An organic EL (Electroluminescence) display device has advantages that it is easily manufactured in a thin plate state, has quick response time, and consumes less electricity due to no need of back light, which is prospective as a display device that replaces a liquid crystal display device and a CRT (Cathode Ray Tube).
Although one piece of film is able to constitute an organic EL layer (emitting layer) in a monochrome organic EL display device, it is necessary to form organic EL layers corresponding to the three primary colors of red (R), green (G) and blue (G) in each pixel in the case of a full-color organic EL display device. In the organic EL display device of 170 ppi (pixel per inch), for example, the size of one pixel is 150 (m(150 (m and one pixel is constituted by three sub-pixels of which each size is 50 (m(150 (m. The three sub-pixels are a red sub-pixel, a green sub-pixel and a blue sub-pixel. Therefore, it is necessary to form each of a red-emitting organic EL layer, a green-emitting organic EL layer, and a blue-emitting organic EL layer with the width of 50 (m.
In recent years, a high resolution display device having 200 ppi (pixel size: 127 (m, sub-pixel width: 42.3 (m) to 500 ppi (pixel size: 50.8 (m, sub-pixel width: 17 (m) has been required. Further, various kinds of screen size from as small as approximately 2 inches to as large as approximately 30 inches have been required.
The organic EL display device is manufactured by forming TFTs (thin film transistor), insulating films, electrodes, and organic EL layers on a substrate called a mother glass. Even in the case of the organic EL display device having a small screen size, a large substrate with the size of approximately 400 m(500 mm to 730 mm(920 mm is used and a plurality of organic EL display devices are simultaneously manufactured on one substrate in order to reduce manufacturing cost. In future, it is expected that a larger substrate than up to now will be used due to the demand of further reduction of the manufacturing cost and a larger screen size.
In the case of low molecular organic EL material, the organic EL layers of three colors are formed in each pixel region by depositing the organic EL material of red emission, green emission and blue emission on the substrate using a shadow mask. However, in polymeric organic EL material, heat decomposes polymer and thus it is impossible to form the organic EL film by a deposition method. For this reason, in general, a coater capable of performing an inkjet method is generally used, and ink which is made of the organic EL material is sprayed in a dotted state onto the substrate by each sub-pixel to form the organic EL layer.
In the coater for the inkjet method, it is necessary to spray a plurality of ink dots in one sub-pixel region. At this point, it is difficult to form the organic EL layer uniformly in the entire sub-pixel region because of overlapping of dots or the occurrence of gap between dots. Consequently, although the coater for the inkjet method can be applied for a display device having the sub-pixel size of approximately 50 (m(150 (m, it may not be applicable for manufacturing a display device of higher resolution.
Further, as the substrate size increases, it is expected that the position of dots shift from a predetermined position due to the affect of thermal expansion of substrate.
Moreover, since the coater for the inkjet method sprays the organic EL material onto all sub-pixels, it has a drawback that it takes more time in proportion to the number of sub-pixels and thus the manufacturing cost increases.